Create System.R

An updated version of System.R.  The sckm function, when used with a "seir" system, now accepts the parameter n.  n should be a named vector with elements "ne" for the number of exposed states, and "ni" for the number of infected states.  Must have at least one infected state.  

Example:  system1 <- sckm("seir", n=c(ne=3,ni=3))